The high melting point and vaporization temperature of tungsten has made it useful for the wire filaments in incandescent electric lamps, discharge lamps, and electrode rods for electric arc welding. Pure tungsten is known to be unsatisfactory for forming wire filaments or electrodes, because tungsten recrystallizes at a relatively low temperature. For example, grain growth throughout the operating life of the pure tungsten filament leads to the development of highly stressed grain boundaries, a high creep rate, offsetting, and early failure of the lamp filaments.
A fine equiaxed grain structure is preferred for dissipating shock and vibration energy during rough service usage of tungsten wire filaments, for example in automobile lamps. Wire comprised of dispersed thoria particles has been used for such applications. The thoria particles maintain the fine grained structure during operation by inhibiting grain growth. Tungsten comprised of dispersed thoria exhibits improved electron emission for the discharge lamp application, and greater vibration resistance to improve life in the lamp application. Tungsten rod electrodes comprised of up to about 4 percent thoria have improved ignition properties greater arc stability, higher current loading capability, and longer life due to the recrystallization inhibiting effect of thoria in the grain boundaries.
However, thoria is a radioactive material, and there is great environmental incentive to replace the thoria in tungsten rods and wires with another additive more compatible with the environment. Preferably, the replacement material will provide the improved electron emission and fine grain equiaxed grain structure having inhibited grain growth found in tungsten comprised of dispersed thoria particles.
U.S. Pat. Nos. 4,923,673 and 4,678,718, and Japanese Kokai Patents 170,844, 143,041, and 286,698 disclose tungsten rods or wires comprised of oxides of either lanthanum, yttrium, or cerium for replacing thoria. The rare earth oxide is dispersed or doped in the tungsten by either mixing a powder of the rare earth oxide with a tungsten oxide powder, or mixing a tungsten oxide powder with a nitrate solution of the rare earth. The nitrate solution mixture is heated in a reducing atmosphere to reduce the tungsten oxide, decompose the nitrate, and form a doped tungsten powder comprised of the rare earth oxide. The doped tungsten powder can be compacted and sintered to form a bar for rod or wire reduction. As used herein, the term "doped" means the intentional addition of an impurity such as ceria in a small controlled amount to improve properties such as creep-resistance or electron emission in articles formed from the doped material.
For those skilled in the art, it is well known that at a given percentage of rare earth dopant in the tungsten, a finer dispersion of the rare earth in the tungsten provides improved rod and wire reducing or drawing properties, improved uniformity of distribution of the particles in the final rod or wire, and improved resistance to recrystallization, grain growth, and grain boundary sliding. In other words, a finer distribution of the rare earth oxide provides a finer grain size tungsten and improved service life in the tungsten wire or rod.
One aspect of this invention is to provide a method for forming a fine dispersion of a metal oxide on tungsten powder.
Another aspect of this invention is to provide a fine dispersion of a metal oxide on tungsten powder by a precipitation process.